Electronically controlled injection devices have recently been developed and put on the market. Such devices are generally adapted to utilize an injection cartridge with one or more chambers, and comprise an electronic control unit which may be programmed to carry out the necessary operations for the administering of injections.
Examples of electronically controlled injection devices according to the prior art are described in EP-A-0 164 539 and EP-A-0 293 958. These references give a good overview of the state of the art in this field.
Electronically controlled injection devices have turned out to have a number of important advantages, especially in those cases where the patient has to administer the injections to himself, as is the case in the ambulatory treatment of diabetics with insulin. The dose to be administered may be set in advance by the physician, and may even be set at a different value depending on which time of the day the dose is to be administered. The timer will give a signal when it is time for administering a dose, and the programmed electric motor will ensure that the correct amount is administered. Through modem electronical engineering and integrated circuitry, the device may be made small enough to resemble a fountain pen, which can easily be carried in an inside jacket pocket or in a lady's purse.
However, there is still room for improvements in this field. It is desirable to let the operations for the administering comprise the zeroing of a counter and a display before and after the administering, the removal of entrapped air from the cartridge, the metering out of doses of a predetermined magnitude, and the giving of a signal when the cartridge is empty. It may also be desirable to provide a timer to give a signal at the proper time for administering an injection, as well as other functions. The device may often also comprise an electrical motor which, on a signal from the control unit, drives a piston rod for a predetermined distance, such that a predetermined dose is administered. Other functions are also possible. This especially applies to the preparation and readying of the injection device before the first administering is to be carried out from a freshly inserted injection cartridge. These preparation and readying steps include the mixing of components to reconstitute an injectable composition in the cartridge, when a multi-chamber cartridge is used, the removal of entrapped gas from the cartridge, and the zeroing of the counter and/or display unit in the control unit before the first administering. If a multi-chamber cartridge is used, the mixing of components to reconstitute an injectable preparation is also included in the readying steps.
A multi-chamber injection cartridge usually comprises two chambers within a cartridge barrel, which chambers are separated by a front piston. The front chamber contains a solid component of an injectable composition, and is closed at its front end by a closure which may be pierced by an injection needle or cannula. The rear chamber contains a liquid component of an injectable composition and is closed at its rear end by a rear piston. To prepare the cartridge for the administering of an injectable composition, the rear piston is moved forward. Through the essentially incompressible liquid in the rear chamber, the front piston will then also be moved forward, and will at a predetermined position activate a bypass connection, such that the liquid in the rear chamber will be urged over into the front chamber to be mixed with the solid component and form a solution or dispersion to be injected. Multi-chamber injection cartridges comprising more than two chambers are also known.
The design and function of single-chamber and multi-chamber injection cartridges are well-known to those skilled in the art, and need not be described here in more detail.
When an injection device is to be readied for the administering of injections, an injection cartridge is first positioned and secured in a predetermined location in the device, and a piston rod is brought into contact with the rear face of the rear piston. By advancement of the piston rod forward, the cartridge is then readied for administering, as is described in the foregoing, to reconstitute the injectable composition. During the reconstitution step, an injection needle may be inserted through the front end closure of the cartridge, to afford a connection with the outer atmosphere and prevent the build-up of an overpressure in the cartridge, but this is not strictly necessary at this stage of the process.
The reconstitution of the injectable composition must often be carried out under mild conditions, so that sensitive compositions, such as growth hormones, are not unduly decomposed. For this, the cartridge should be held vertically with its front end pointing upward, and the liquid component should be made to flow calmly into the front chamber without any shaking or agitation to aid in the dissolution of the solid component. Other reconstitution positions, such as vertical with the front end pointing downwards or horizontal, may be utilized, depending on e.g. distribution of the solids or by-pass design and location.
Generally when mixing components it may also be desirable to secure wetting of all solids and all wall surfaces or to impose a certain agitation by turning or rotating the device before subsequent steps in the administration scheme.
The next step in the readying of the injection device is the removal of bubbles of air and other gases from the cartridge. This is an important step, even though the amount of enclosed gas in modem injection cartridges is usually not sufficient to cause a health hazard of air embolism if it is accidentally injected. However, as the entrapped gases are compressible, they serve as a buffer, which may give rise to inaccuracies in the metering out of the liquid composition.
The removal of air and other gases is usually carried out in the conventional way: After having inserted an injection needle through the front closure of the cartridge (if this has not been done previously), the user holds the injection device vertically with the needle pointing upward, and advances the piston rod forward until no gas, but only liquid comes out through the needle.
After the gas has been expelled from the cartridge, the metering scale and its display is set at zero or some other desired value before the administering of the injectable composition is started.
In electronically controlled injection devices, the reconstitution of the injectable composition, the removal of enclosed air and the zeroing of the metering scale are usually carried out automatically when the user pushes a start button or the like.
From what has been stated above, it follows that the process of readying an injection device for the administering of injections is a rather delicate process, which demands considerable care from the user. However, this is often forgotten by the user, who usually has no medical training. Thus, the reconstitution of the injectable composition is often carried out with the cartridge in any arbitrary orientation, which may lead to an undesired decomposition of a sensitive composition. The removal of air may be forgotten completely, or may be carried cut with the needle pointing in an unsuitable direction, which may lead to an ineffective removal and to spillage of the preparation.
Also during the administration process the device orientation may be critical. Some injection positions may be unsuitable and indicative of improper use. Inadvertent administration of multiple doses may take place unless the device has been moved or turned between two consecutive injections.
Through the present invention, the above shortcomings are eliminated.